A few ideas:
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How did you determine your FTP? Your FTP may be set too high, in which case the 10 min interval above is more like a solid VO2max interval instead of a slightly-above-threshold interval. This was the case for me. I now take the ramp test result, and reduce by 5%.
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Your limiter may be your cardiac capacity, not your muscles. I am in this same boat. I have never paused a workout or reduced the workout intensity due to leg fatigue. It has always been because my heart can’t pump enough. There are a variety of reasons for why this might be the case. For me, I think I just have a small heart, and may also have some mild atherosclerosis (a number of medical tests over the years have led me to this conclusion). Echoing a few of the posts above, worth going to a doc to get some heart diagnostics done (eg stress test, EKG, calcium scan).
FWIW, about 80% of people are limited by muscle fatigue in a ramp test. 20% by heart/breathing.